1. Technical Field
The present invention relates to an oscillation circuit for generating an oscillation signal, a test apparatus including the oscillation circuit, and an electronic device including the oscillation circuit. In particular, the present invention relates to an oscillation circuit for generating an oscillation signal synchronized with a supplied reference clock.
2. Related Art
Conventionally, a PLL (Phase Locked Loop) circuit has been known as a circuit for generating an oscillation signal. The PLL circuit includes a voltage control oscillator such as a ring oscillator, a phase comparator for comparing the phases respectively of the reference clock and the oscillation signal outputted from the voltage control oscillator, a charge pump for outputting a control voltage responding to the phase comparison result, and a filter for controlling the oscillation frequency of the voltage control oscillator by passing the control voltage.
The difference between the period in each cycle of an oscillation signal outputted from the voltage control oscillator and the period of the signal to be generated is accumulated each time the signal circulates a loop circuit in the voltage control oscillator. The phase comparator detects the accumulated phase shift for each pulse of a reference clock. The charge pump and the filter generate a control voltage that causes the average of the accumulated phase shift to be substantially 0. Currently no related patent document has been recognized, and so the description thereof is omitted.
However, as described above, a conventional PLL circuit sends feedback so as to cause the average of the accumulated phase shift to be substantially zero. Therefore, there still exists a difference between the average of the phase deviation and the actual phase deviation in feedback, as a phase error.
For this reason, the conventional PLL circuit cannot generate an oscillation signal having the phase controlled with high accuracy. In a test apparatus for testing a device under test such as a semiconductor circuit, a clock generating circuit is used for generating a clock determining the operation of the test apparatus. When the above-described PLL circuit is used as such a clock generating circuit, the PLL circuit cannot test the device under test with high accuracy.
A PLL circuit is also provided in an electronic device such as an IC chip, for the purpose of generating a clock determining the operation of the electronic device. However, since the conventional PLL circuit cannot control the phase with high accuracy as described above, the electronic device will operate unstably. In addition, the charge pump and the filter require a large area for the circuits thereof, and therefore the conventional PLL circuit has a large circuitry dimension.